Unbounded operators commuting with the commutant of a restricted backward shift

Unbounded Commuting Operators and Multivariate Orthogonal Polynomials

The multivariate orthogonal polynomials are related to a family of operators whose matrix representations are block Jacobi matrices. A sufficient condition is given so that these operators, in general unbounded, are commuting and selfadjoint. The spectral theorem for these operators is used to establish the existence of the measure of orthogonality in Favard's theorem.

Operators commuting with the shift on sequence spaces

An invariant for pairs of almost commuting unbounded operators

For a wide class of pairs of unbounded selfadjoint operators (A,B) with bounded commutator and with operator (1+A2+B2)−1 being compact we construct aK-theoretical integer invariant ω(A,B), which is continuous, is equal to zero for commuting operators and is equal to one for the pair (x,−i d/dx). Such invariants in the case of bounded operators were constructed by T. Loring [1, 3] for matrices r...

Hypercyclic Operators That Commute with the Bergman Backward Shift

The backward shift B on the Bergman space of the unit disc is known to be hypercyclic (meaning: it has a dense orbit). Here we ask: “Which operators that commute with B inherit its hypercyclicity?” We show that the problem reduces to the study of operators of the form φ(B) where φ is a holomorphic self-map of the unit disc that multiplies the Dirichlet space into itself, and that the question o...

Commutant Lifting for Commuting Row Contractions

The commutant lifting theorem of Sz.Nagy and Foiaş [22, 21] is a central result in the dilation theory of a single contraction. It states that if T ∈ B(H) is a contraction with isometric dilation V acting on K ⊃ H, and TX = XT , then there is an operator Y with ‖Y ‖ = ‖X‖, V Y = Y V and PHY = XPH. This result is equivalent to Ando’s Theorem that two commuting contractions have a joint (power) d...